The present invention relates to a controller suitable for use at low temperatures or high temperatures.
For example as a controller for use at low temperatures as attached to a vacuum tank, heretofore known is an axially elongated valve which comprises a valve case to be disposed inside the tank, an actuator to be provided outside the tank, and an extension pipe for holding the valve case in communication with the actuator.
This valve has the problem that the extension pipe needs to be given a particular length in conformity with the design of piping when the valve is to be fabricated, further making the vacuum tank larger in size.
To overcome this problem, a controller has been proposed which is adapted for use with its actuator entirely placed in the vacuum tank.
The proposed controller comprises a valve case opened upward, a gas-driven actuator having a fixed portion provided on the valve case and fixed thereto and an operating portion movable upward and downward by a drive gas when the gas flows in or out, a valve stem extending upward from inside the valve case to above the case and having its upper end fixed to the operating portion of the actuator, and a valve element mounted on the lower end of the valve stem. The controller has a valve case fluid seal bellows extending between and attached to the valve stem and the fixed portion of the actuator, and an actuator gas seal bellows extending between and attached to the fixed portion and the operating portion of the actuator.
The conventional controller described has a closed space formed by the valve stem, fluid seal bellows, actuator fixed portion, gas seal bellows and actuator operating portion, and air is confined in this closed space. The pressure of the air in the closed space varies with the variation of the ambient temperature, consequently moving the valve stem. Use of the controller as a flow regulating valve therefore involves the fatal problem that the movement of the valve stem varies the flow rate of fluid through the valve case. The same problem is encountered also in the case where the controller is used at high temperatures.